The application of substances to the skin with the objective of enhancing its appearance has been practiced for thousands of years. Such substances are usually applied in liquid or semisolid forms and include a large number of materials of plant, animal or synthetic origin.
Anatomically the skin can be described as a stratified organ with three distinct tissue layers: the epidermis, the dermis and the subcutaneous fat layer.
The epidermis, the outermost skin layer is comprised of stratified squamous epithelial cells. On the surface of this layer, the flattened remnants of the dividing cells which originate from the lower basal area accumulate forming a relatively thin coating which serves as a barrier between the integument and the environment. This outermost area which is about 10.mu. in thickness, is known as the stratum corneum and is very rich in keratin, a protein that contributes to the cytoskeleton of the epidermal cells. The stratum corneum has the capacity to retain water to a much greater extent than other keratinized tissues such as nails and skin. This water retention is temporary, since water readily evaporates due to the physical location, in contact with the external environment, and body temperature. The stratum corneum serves as a protective physical and chemical barrier for the body preventing the penetration of most substances via the skin.
Dry, rough and pigmented skin is a common occurrence associated with the aging process. As skin ages, it thins and hygroscopic substances decrease reducing the ability of the skin to retain water. Hormonal changes that accompany aging result in lower sebum output and therefore lowered skin lubrication. In addition, keratin cross-linking induced by long term exposure to UV radiation causes skin to harden.
Water diffuses rapidly to the keratin layer from the lower skin layers, at a rate about 50-100 times faster than it is lost by evaporation. Water movement through the keratin layer is rather slow due to the intercellular lipid component that surrounds the keratinocytes (where the keratinocytes can be considered the bricks and lipids the mortar).
In the skin there are natural water retaining, or moisturizing factors which can be easily removed by excessive washing, leading to a dry skin condition.
Substances applied on the skin can alter the skin surface if their penetration is blocked by the stratum corneum by forming a cuticle or film. This film can act as an antimicrobial if the substance applied has antimicrobial properties, can protect against sun damage if it contains a sunscreen or can have cleansing effects if it contains surfactants.
Films can display different degrees of occlusiveness providing a moisturizing effect, not due to their water content, which as mentioned would rapidly evaporate, but by preventing the loss of water from the underlying skin.
The dry stratum corneum therefore becomes hydrated as the water is delayed in evaporating, and this increased moisture results in the skin feeling softer and attaining a transient smoother appearance. Since such barriers are hydrophobic in nature, they tend to generate an oily or shiny appearance and therefore can only be used in moderation and combined with hydrophilic substances, which by definition will restore water loss causing the skin to regain its original appearance.
Because of the natural protective function of the skin most substances are precluded from penetrating across the stratum corneum. However, this protective effect also precludes the penetration of macromolecules such as collagen, elastin, glycosaminoglycans, and other desirable substances which are lost from the underlying dermis during the process of aging. Thus, the application of such substances to the skin in the form of creams or ointments becomes a futile effort, since they cannot penetrate and become incorporated into the tissue. At the best they will generate a partially occlusive coating that will alter the natural appearance of the skin.